Ethanol production by fermentation has become a major source of renewable fuel for transportation. Yeast fermentation of sugars derived from sugar cane, corn, sorghum or sugar beet feedstock and recovery of ethanol from the fermentation mash by distillation is used industrially for producing ethanol as gasoline additive or transportation fuel. Recovering yeast from the fermentation mash by centrifugation and recycle of recovered yeast to the fermentation can be used to increase fermentation rates and ethanol productivity. Byproducts from the fermentation, such as dried distillers grains and solubles (DDGS) from corn based fermentation or dried yeast from sugar cane based fermentation, are used as animal feed or feed additives.
A common problem in the production of fuel ethanol by fermentation is contamination of the fermentation mash with bacteria, in particular with lactic acid and acetic acid producing bacteria, which can lead to lowered ethanol yield and can interfere with yeast recycle by causing yeast flocculation. Antibiotics that act selectively on bacteria have been used to reduce bacterial contamination, but the use of such antibiotics is undesirable because it leads to contamination of fermentation byproducts with antibiotics and can lead to development of antibiotics resistance in bacteria.
An acid treatment lowering the pH to less than 2.5 is commonly applied to reduce bacterial contamination of recovered yeast before recycling it to fermentation. However, such an acid treatment is not sufficiently effective against acid tolerant bacteria, such as lactic acid and acetic acid producing bacteria.
WO 2011/116042 proposes to control growth of bacteria in an ethanol fermentation system by carrying out fermentation in the presence of a nonoxidizing biocide or a stabilized oxidizer, such as stabilized peracetic acid. The nonoxidizing biocide or stabilized oxidizer is added to the fermentation vessel.
WO 2012/027469 proposes to reduce bacteria levels in a fermentation system by introducing an organic oxidizing compound, such as peracetic acid, and an inorganic oxidizing compound, such as hydrogen peroxide, into a fermentation mash.
US 2009/0061490 discloses fermenting xylose in a clarified sugar hydrolysate, where a yeast slurry is separated from the fermentation broth, treated with an oxidant to reduce microbial contaminants and re-introduced into the fermentation. Treatment with the oxidant is carried out in the pH range between 3 and 6.